The invention relates generally to separating carbon dioxide (CO2), nitrogen oxides (NOx), sulfur oxides (SOx), mercury and other air pollutants from flue gas, coal gas, and other gas mixtures, using a multifunctional filter filled with a carbon-rich sorbent.
Flue gas is a byproduct of fossil-fuel combustion, for example coal combustion in power plants. Flue gas contains mostly nitrogen but also some CO, unreacted oxygen, water vapor, about 10-15% of CO2, and parts-per-million levels of NOx, SOx and other pollutants, such as mercury and arsenic compounds. Other large-volume gas mixtures, such as coal gasification streams, can also contain CO2 that needs to be recovered. The incentives to recover CO2 are two-fold: to produce and utilize CO2, and to capture and store the unused portion of it that otherwise pollutes the environment.
CO2 can be recovered from flue gas using conventional liquid absorption, membrane separation and solid adsorption. The benchmark commercial approach is the aqueous amine absorption that produces CO2 for about $40-50/ton. The other known approaches can be even more expensive. For example, membrane separations usually involve high compression costs. Solid pressure-swing adsorption, on the other hand, requires expensive and often moisture sensitive materials, such as zeolites (R. V. Siriwardane et al. Energy Fuels 15, 279-284, 2001), high compression costs, and expensive sorbent recovery. This includes alkali-metal-based and amine-based sorbents, virgin or deposited on porous materials, which capture CO2 via carbonate-forming chemical reactions (for example, S. C. Lee et al. Catalysis Today, 111, 385-390, 2006; N. Shigemoto et al. Energy Fuels, 20, 721-726, 2006; A. Arenillas et al. Fuel, 84, 2204-2210, 2005).
On the basis of prior research related to displacing coal-bed methane with CO2, we know that virgin coal has a higher capacity for CO2 than it does for methane (for example, A. Nodzenski, Fuel 77, 1243-1246, 1998). This has been demonstrated at elevated pressures relevant to enhanced coal-bed-methane recovery. It is also known that activated carbon can have even higher CO2 capacity. Again, this has been established at elevated pressures relevant to pressure-swing adsorption (PSA) (Na et al. Ind. Eng. Chem. Res. 41, 5498-5503, 2002; R. V. Siriwardane et al. Energy Fuels 15, 279-284, 2001). Separately, carbon-rich materials tend to exhibit low CO2 sorption heats but are known to absorb microwave energy, for example to heat up a spent sorbent to desorb or react the sorbed material (Yuan et al. Proceedings of Air & Waste Management Association 94th Annual Conference, Orlando, 2001; Kong and Cha, Energy Fuels, 10, 1245-1249, 1996; 9, 971-975, 1995). There are also numerous references to conventional adsorption on solids and absorption in liquids aimed at separating gas mixtures. A sample of relevant references is given below.
U.S. Pat. No. 7,153,344 concerns a method of removing CO2 from the gas produced by oxidative combustion of carbon-containing fuels using a “semipermeable material”, such as a hollow-fiber membrane or molecular sieve adsorbent, which allows for passage of nitrogen, but retains oxygen containing components, mainly CO2, H2O and O2. By contrast, the process disclosed in this invention does not utilize membranes or H2O— or O2-retaining molecular sieves. Instead, this invention utilizes multifunctional carbon-rich sorbents that, while CO2 specific, largely allow for passage of H2O and O2.
US Patent Application 2006/0162556 A1 and PCT Application WO 03/053546 teach an adsorptive separation method and apparatus to purify air (not flue gas) prior to its cryogenic separation, by removing primarily nitrous oxide (N2O) but also possibly other air impurities, such as hydrocarbons, CO2 and water, on a moisture-retaining zeolite-type adsorbent or activated carbon. By contrast, the process disclosed in this invention deals with removal of CO2, not from air, and does not utilize materials that retain moisture.
PCT Application WO 2005/108297 A2 teaches a method to capture excess CO2 from ambient air (not from flue gas) using a variety of methods primarily based on a chemical reaction of CO2 with common minerals to produce solid carbonate materials.
U.S. Pat. No. 6,562,103 B2 teaches a pressure-swing adsorption (PSA) process for direct reduction of iron where CO2 is removed from the spent reducing gas.
European Patent EP 0 768 117 B1 teaches a method how to prepare a carbonaceous material and how to modify it by halogenation and hydrocarbon treatment to make it a suitable pressure-swing adsorbent for separating air into oxygen and nitrogen.
European Patent EP 0 636 672 B1, U.S. Pat. No. 5,522,228, and WO 95/04115 disclose a refrigeration process by adsorption and desorption of pure CO2 on solid surfaces of activated carbon fibers or an active charcoal at elevated pressures.
U.S. Pat. No. 5,439,054 teaches an in-situ method of separating gaseous fluids, for example mixtures containing methane and CO2, within a carbonaceous subterranean material, such as a coal seam.
U.S. Pat. No. 4,770,676 discloses a pressure-swing adsorption (PSA) and temperature-swing adsorption (TSA) process to recover methane from land fill gas.
U.S. Pat. No. 4,624,839 discloses a liquid absorption of CO2 in an aqueous amine solution improved with a copper inhibitor additive.
The objective of this invention is to use an inexpensive and relatively moisture-insensitive sorbent that can work at low pressures, without the need for feed compression, has relatively low heat of adsorption for an efficient thermal recovery, and, ideally, captures primarily CO2, residual NOx, SOx (if needed), and mercury, but does not retain much nitrogen, oxygen, and water. Carbon-rich sorbents, for example carbonaceous sorbents derived from coal and other carbon-containing materials, meet this requirement.